Conventionally, mainly nickel-cadmium batteries have been used as secondary batteries for memory-backup of AV and information devices, such as personal computers, VTRs, and the like, or as power sources for driving these devices. Because of their high voltage/energy concentration and excellent self-discharge characteristics, non-aqueous electrolyte secondary batteries have particularly been attracting attention as a replacement for nickel-cadmium batteries. As a result of various development efforts, some non-aqueous electrolyte secondary batteries have already been commercialized. Currently, more than half of notebook computers, cellular phones, and the like are driven by non-aqueous electrolyte secondary batteries, and there are high expectations for their use in the near future in environmental vehicles, of which electrical vehicles and hybrid vehicles are representative.
Carbon is often used as a negative electrode material in these non-aqueous electrolyte secondary batteries, and various organic solvents are used in the electrolytes in order to mitigate the risk when lithium is produced on the surface of the negative electrode, as well as to achieve a high driving voltage. Particularly in non-aqueous electrolyte secondary batteries for cameras, alkali metals (especially lithium metals or lithium alloys) and the like are used as the negative electrode material, and aprotic organic solvents such as ester organic solvents are normally used in the electrolytes.
As described above, non-aqueous electrolyte secondary batteries exhibit high performance but cannot be considered to exhibit sufficient safety. First, alkali metals (especially lithium metals, lithium alloys, and the like) that are used as negative electrode material for non-aqueous electrolyte secondary batteries have extremely high activity with respect to water. Therefore, when for example water enters through an imperfect seal in the battery, the negative electrode material and the water react yielding hydrogen, which poses the risk of ignition or the like. Furthermore, since lithium metal has a low melting point (about 170° C.), an extremely dangerous situation may occur, such as the battery melting when a large current suddenly flows during a short circuit or the like, causing the battery to heat excessively. Moreover, the danger exists of the battery exploding or igniting due to gas that is generated upon the electrolytes evaporating or decomposing due to generation of heat in the battery. It has also been pointed out that the battery may be contaminated by minute scrap metal at the time of electrode composition, leading to a short circuit, abnormal heat generation, or ignition.
In order to solve the aforementioned problems, when temperature increases and pressure inside the battery rises during a short circuit or overcharge of a cylindrical battery, for example, a technique has been proposed to provide a mechanism in the battery whereby an excessive current of a predetermined amount or greater is prevented from flowing into the cylindrical battery by breaking electrode terminals at the same time as when a safety valve operates (see NPL 1). The mechanism does not necessarily operate normally, however, and when the mechanism does not operate normally, more heat is generated due to the excessive current, and thus the danger of ignition or the like still remains. Therefore, there is demand for the development of a non-aqueous electrolyte secondary battery that fundamentally reduces the risk of electrolyte evaporation or decomposition, ignition, and the like without relying upon a safety mechanism such as a safety valve.
A non-aqueous electrolyte secondary battery that greatly reduces the danger of the battery igniting and burning in an emergency, such as a short circuit, has thus been developed by adding a phosphazene compound to the non-aqueous electrolyte to make the non-aqueous electrolyte incombustible, flame retardant, or self-extinguishing (see PTL 1).